1. Field of the Invention
This invention relates to apparatus for measuring alternating current accurately and more particularly to such apparatus which accurately measures currents over a wide range.
2. Background Information
A current transformer provides a secondary winding current proportional to the current flowing through its primary winding. One application of current transformers is in the accurate reproduction of ac current conditions in a power system for current sensing and operation of protective relays.
The inverse-time and instantaneous overcurrent relay is one type of relay that uses current transformers. It must detect an input current at a certain level (called "pickup") and either perform an inverse-time protection function based upon current above pickup or provide an instantaneous protection function for currents above pickup.
The pickup may vary widely and the required working range could be large. For example, in one type of overcurrent relay, the pickup range is 0.5 A to 12 A and the current range is 40.times. pickup with a ceiling of 240 A. This means that the device must be able to handle accurately a current range of 0.5 A to 240 A or 480:1.
In the prior art, accommodating this wide range of input currents is accomplished by either changing the winding ratio of the current transformer or by changing the burden resistor in the secondary as a function of pickup level. In either case, the net effect is to provide a fixed voltage across the secondary for a given pickup level.
A change in burden resistors is achieved by a number of precision resistors in series with switches or relays and a change in the winding ratio is achieved by tap blocks that manually change the number of primary windings.
There remains a need for apparatus which can accurately measure a wide range of input currents without the need for switching burden resistors or modifying the current transformer winding ratio.